There are two stones left at Marnoch church. One stands at over 8 feet at the churchs front door within the walls and another stands just north of the church at about 6 feet. However another stone lies fallen at the back of the wall nearer the house. (I see no mentions of this stone and assume that it has always been there) Marnoch Church is signposted and can be found just north of the B9117. Leaving the A97 it is about 1 mile.
A whopping single standing stone in a tiny churchyard is all that is left of what was perhaps once a stone circle. This is one of those crazy stones that you don't realise how tall it is until you stand right next to it and it towers above you!
Canmore's record tells us that this 8ft+ standing stone called St Marnan's Chair is now within the walls of a churchyard, at NJ 59705020. There is another, smaller stone (perhaps moved from its original position SE of the Chair) at NJ 59715024.
The entry states the Chair "is almost certainly all that remains of a stone circle, probably one of the pillar stones of a recumbent stone circle centred to the NE where the church stands."
A little further afield, in Banffshire, just beyond the Aberdeenshire boundary, two standing-stones represent all that remains of a circle which stood on the site of the present parish church of Marnoch. The taller of the two is known as St Marnan's Chair, though it bears no resemblance to such an article of furniture in its present state. As, however, it seems likely to have been one of the pillar-stones associated with the recumbent stone, its original position may have given some excuse for the designation. St Marnan, or Marnoch, was a seventh-century missionary who is said to have died at his church here in 625.
This is from Ritchie, J., Folklore of Aberdeenshire Stone Circles, in Proc. Soc. Ant. of Scotland, LX, 1926, pp304-313.
Naturally the chair is where St Marnan (or St Marnoch, the name of the village/church) is said to have preached. He is associated with a curious bit of folklore - that his skull was taken from the church and washed every Sunday in his renowned (but now defunct) holy well near the river, the washing water being given to the sick*. One might be tempted to think of 'Celtic head cults' and the like. I don't think drinking water out of a skull would make me feel any better, personally.